Repair Specifications
- Difficulty: Intermediate
- Estimated Time: 45 – 90 Minutes
- Tools Needed: 1/4″ Nut Driver or Phillips Head Screwdriver, Digital Multimeter, New High-Efficiency Air Filter, Flashlight.
- Estimated Cost: $20 – $100 (Depending on if parts like the switch or filter need replacement).
Lennox Error E250 is a critical safety lockout indicating that the furnace’s primary limit switch has opened multiple times within a single heating cycle. This “Limit Switch Open Lockout” occurs when the control board detects an over-temperature condition, tripping the safety circuit to prevent catastrophic heat exchanger damage or potential fire hazards.
🛑 Important: Official Documentation
Before unscrewing any panel, ensure you have the correct service manual for safety.
If you are facing this error, you likely noticed your furnace suddenly stop producing heat, followed by the blower fan running continuously in an attempt to cool down the internal components. You might hear the unit trying to ignite and then clicking off, or feel cold air blowing through your vents. While a lockout sounds intimidating, it is a protective measure designed to keep your home safe. Rest assured, with a systematic approach to restoring airflow and checking electrical continuity, this is a repair that can often be resolved without a costly emergency service call.
Troubleshooting & Replacement Instructions
Troubleshooting & Replacement Instructions
Follow these steps in order. Most E250 errors are solved in the first three steps.
Step 1: Complete Power Shutdown. Before touching any internal components, turn off the furnace at the dedicated circuit breaker and the service switch located on the side of the unit. Safety Warning: Furnaces carry high-voltage electricity and sharp metal edges. Always ensure power is verified OFF with a non-contact voltage tester before reaching inside.
Step 2: Inspect and Replace the Air Filter. Remove your air filter and hold it up to a light source. If you cannot see light through it, it is restricted. A clogged filter is responsible for roughly 80% of E250 codes. Even if it looks “okay,” try running the furnace briefly without the filter to see if the error clears (do not do this for more than 10 minutes to avoid getting dust on the motor).
Step 3: Reset the Lockout. The E250 is a “hard” lockout, meaning it won’t always clear itself just by fixing the filter. With the power off, wait at least 30 seconds, then flip the power back on. This resets the control board’s memory. If the error returns immediately, the limit switch is likely still “open” (hot) or faulty.
Step 4: Locate and Test the Limit Switch. Remove the upper cabinet panel. The primary limit switch is usually located on the center of the heat exchanger wrap-around. It is a small circular or rectangular component with two wires attached. Remove the wires (pull the spade connectors gently). Set your multimeter to the Ohms (Ω) setting. Place the probes on the two terminals of the switch.
Reading 0.0 to 0.5 Ohms: The switch is closed (Good).
Reading ‘OL’ or Infinite: The switch is open (Bad/Tripped). If the furnace is cool and the reading is OL, the switch is defective and must be replaced.
Step 5: Inspect the Blower Assembly. If the switch tests fine, check the blower wheel. Reach in (power OFF!) and try to spin the blower wheel. It should spin freely. If it feels stiff, the bearings are shot. If it is covered in thick dust, use a vacuum and a small brush to clean the “fins” of the squirrel cage. Ensure all supply registers in the house are open; closing more than 20% of your registers can cause the E250 error.
Step 6: Installing a New Switch (If necessary). If the switch tested “Open” while cool, unscrew the two mounting screws using your 1/4″ nut driver. Pull the old switch out. Ensure the new switch has the same temperature rating (printed on the side of the part, e.g., “L180-20”). Slide the new switch in, tighten the screws, and reconnect the two wires. There is no polarity, so it doesn’t matter which wire goes to which terminal.
Detailed Diagnosis: Root Causes
Detailed Diagnosis: Root Causes
To fix Error E250, we must understand why the limit switch tripped. The limit switch is a normally-closed (NC) sensor that opens when internal temperatures exceed a safe threshold. When it opens too many times, the E250 lockout is triggered.
1. Restricted Airflow (The #1 Cause)
The heat exchanger requires a constant flow of cool air to strip away the heat it generates. If the air is blocked by a heavily soiled filter, closed supply registers, or a blocked return air duct, the heat builds up inside the cabinet until the limit switch “pops.” Over time, dust buildup on the secondary heat exchanger or the A-coil (if you have AC) can also cause this restriction.
2. Blower Motor or Capacitor Failure
Even if the path is clear, if the blower motor isn’t spinning at the correct RPM, the air won’t move fast enough. This can be caused by a failing run capacitor (which reduces motor torque) or “caking” of dust on the blower wheel blades. If the blades are heavy with dirt, they lose their aerodynamic efficiency, leading to higher internal temperatures.
3. Component Fatigue (The “Weak” Switch)
Limit switches are mechanical bimetal discs. Every time they trip and reset, the metal undergoes stress. Eventually, the switch may become “weak,” meaning it trips at a lower temperature than its factory rating (e.g., tripping at 140°F instead of 180°F). Voltage spikes from the control board can also occasionally damage the internal contacts of the switch, causing it to fail in the open position.
4. Gas Manifold Pressure Issues
If the gas valve is “over-firing” (sending more gas than the furnace is rated for), the flame will be too hot for the airflow to handle. This is often the result of a recent DIY gas valve adjustment or a change in gas utility pressure without recalibrating the furnace.
Symptoms of a Lennox E250 Error
Recognizing the physical signs of an E250 lockout can help you differentiate it from simple thermostat issues or gas supply problems. Look for the following indicators:
- Continuous Blower Operation: The most common sign is the indoor blower fan running indefinitely, even when the thermostat is set to “Auto” or the heat is turned off. This is the furnace’s “Cool Down” mode.
- The “Cold Blast”: You feel air moving through the registers, but it is room temperature or cold. This happens because the burners have been disabled while the fan remains active.
- Control Board Display: If you have an iComfort thermostat, the code “E250” will be displayed. If you are looking at the furnace control board through the sight glass, you will see a specific LED flash pattern (consult your manual for the specific blink rate, but usually, it involves a rapid red/yellow sequence).
- Short Cycling: Before the hard lockout occurs, you may have noticed the furnace running for 3–5 minutes, shutting down, and then trying to restart shortly after.
How to Prevent Error E250
Preventing this error is significantly cheaper than replacing a heat exchanger that has cracked due to overheating.
- Strict Filter Schedule: Use a high-quality pleated filter but avoid the “Ultra-Allergen” filters (MERV 13+) unless your ductwork is specifically designed for them, as they can be too restrictive. Change them every 60-90 days.
- Annual Blower Cleaning: Once a year, use a vacuum to clear the dust from the blower motor vents and the control board area. Dust acts as an insulator and causes components to run hotter than intended.
- Static Pressure Check: Have an HVAC technician perform a “Static Pressure Test.” This measures how hard your fan has to work to push air. If the pressure is too high, your ductwork might be undersized, which will lead to chronic E250 errors regardless of how clean your filters are.
Frequently Asked Questions
Q: Can I bypass the limit switch to get heat tonight?
A: No. Under no circumstances should you jumper or bypass a limit switch. This switch is the only thing preventing the furnace from melting or starting a fire if the fan fails. If the switch is open, it is because there is a dangerous heat condition.
Q: How long does the E250 lockout last?
A: Most Lennox boards will attempt a self-reset after 1 to 3 hours of “cool down” time. However, if the underlying cause (like a dirty filter) isn’t addressed, the furnace will simply trip again and return to lockout within minutes.
Q: My filter is clean, but I still get E250. What else could it be?
A: Check your AC evaporator coil (the “A-coil”) located above the furnace. If you have pets or don’t use filters regularly, the underside of this coil can become matted with fur and dust, completely blocking the air from leaving the furnace, regardless of how clean the furnace filter itself is.